Unravelling long-term temporal dynamics of the invasive fruit fly Bactrocera dorsalis along an altitudinal gradient in Morogoro, Eastern Tanzania

Bactrocera dorsalis (Hendel) is the most economically important invasive fruit fly in sub-Saharan Africa, yet long-term empirical evidence linking its seasonal dynamics to environmental drivers across altitudinal gradients remains limited. We analysed 279 site-month observations of male trap catches (flies per trap per day, FTD) collected between October 2004 and February 2012 at six sites spanning 526–1,650 m above sea level along the Uluguru Mountain transect in Morogoro, Tanzania. Climatic variables (temperature, rainfall, relative humidity) were obtained from ERA5 reanalysis, and host fruit phenology was characterised for key commercial species. Abundance declined sharply with altitude, with mean FTD decreasing from 87.1 at the lowland site (SUA, 526 m) to 0.07 at the highest site (Nyandira, 1,650 m), where 55% of months recorded zero catches. Seasonal peaks occurred during the warm–wet period (November–April), but both their magnitude and duration declined with altitude, with the high season shortening from seven months at SUA to three months at Nyandira. Seasonal patterns also became less predictable at higher elevations, with the coefficient of variation of peak abundance increasing from 0.54 to 1.38. Relative humidity showed the strongest association with FTD (Spearman ρ = 0.202, p = 0.0007), while temperature showed no significant overall relationship. Host fruit availability further shaped population dynamics, with ripening of jew plum, mango, and soursop associated with increased abundance, whereas citrus and loquat had minimal influence. Overall, B. dorsalis abundance along the gradient is primarily structured by altitude and host availability, with climate acting as a secondary, context-dependent influence. These findings provide a clearer ecological basis for predicting pest pressure and improving management strategies across heterogeneous landscapes in East Africa.